An abrasive tool is fabricated by compacting a moulding composition to impart the geometrical form of the tool in question, drying the moulding composition thus formed, and firing it, as a concluding step. While the abrasive tool is fired at a high temperature (1250.degree. to 1300.degree. C.), the ceramic bond will melt and pass, completely or partically, into a liquid state to coat and envelope the abrasive synthetic corundum grains and to bond together said abrasive grains while subsequently cooling down and solidifying as the temperature is decreased. The moulding, drying, and firing conditions for moulding compositions in abrasive tooling manufacturing processes are generally known.
A moulding composition should meet the following basic requirements:
abrasive tooling produced from said moulding composition should have high mechanical strength;
abrasive tooling that can be produced from said moulding composition should have a wide hardness range;
the amount of harmful substances that are evolved into the environment during firing operations in the process of abrasive tooling manufacture should be as low as possible; and
the moulding composition should be relatively inexpensive and readily available.
The hardness range of abrasive tooling will depend upon the melting temperature of the bonding agent, hence on its composition. As is generally known, in the tool preform firing process the bonding agent dissolves the corundum grains to increase its viscosity. On the other hand, it is known that a larger amount of bond is required to obtain higher hardness in abrasive tooling. However, increasing bond concentration in a moulding composition will lead to the tool preform being deformed in the firing process before the bond has increased in viscosity sufficiently to prevent deformation.
The above requirements that a moulding composition should meet depend in a large measure upon the composition of ceramic bond, which typically includes plasticizer to prevent abrasive tool deformation while fired, ingredients to enhance strength properties in the bonding agent, and a reactive ingredient to assure adhesion for corundum grains. Boron-containing frits are used as reactive ingredients. The frits are named to reflect the presence of oxides that determine their properties.
There is known a moulding composition to produce abrasive tooling (SU, A, 634923), which contains synthetic corundum material and a bonding agent. The bonding agent comprises kaolin as plasticizer, talc as an ingredient to increase bond strength properties, and a borosilicate frit as reactive ingredient. The proportions of the bond ingredients, in per cent by weight, are as follows:
kaolin: 40 to 95 PA1 borosilicate frit: 4 to 40 PA1 talc: 1 to 20 PA1 silicon oxide: 67.8 to 70.0 PA1 aluminium oxide: 3.3 to 3.6 PA1 magnesium oxide: 0.77 to 0.9 PA1 calcium oxide: 0.53 to 0.6 PA1 iron oxide: 0.1 to 0.23 PA1 sodium oxide: 3.1 to 3.6 PA1 potassium oxide: 4.0 to 4.8 PA1 boron oxide: 15.6 to 17.7 PA1 lithium borosilicate frit: 15.0 to 60.0 PA1 fireclay: 0.1 to 40.0 PA1 feldspar: 15.0 to 60.0 PA1 cryolite: 5.0 to 15.0 PA1 silicon oxide: 64.0 to 75.0 PA1 boron oxide: 10.0 to 18.0 PA1 aluminium oxide: 2.0 to 8.0 PA1 iron oxide: 0.1 to 0.5 PA1 magnesium oxide: 1.0 to 2.0 PA1 calcium oxide: 0.1 to 1.0 PA1 sodium oxide: 3.5 to 6.4 PA1 potassium oxide: 3.5 to 4.6 PA1 lithium oxide: 4.0 to 6.0 PA1 synthetic chrome-titanium corundum: 69 to 90 PA1 kaolin: 1.4 to 5.0 PA1 feldspar: 0.7 to 2.5 PA1 lithium borosilicate frit: 2.1 to 7.5 PA1 cryolite: 0.7 to 2.5 PA1 adhesive and moistening ingredient (water glass): 3.0 to 6.0 PA1 barium silicate frit: 2.1 to 7.5 PA1 synthetic corundum material: 76.5 to 90.0 PA1 kaolinite: 2.1 to 5.7 PA1 feldspar: 2.8 to 7.6 PA1 adhesive and moistening ingredient: 3.0 to 4.5 PA1 calcium borosilicate frit: 2.1 to 5.7 PA1 silicon oxide: 22.0 to 38.0 PA1 aluminium oxide: 14.5 to 15.5 PA1 boron oxide: 25.0 to 35.0 PA1 calcium oxide: 12.5 to 17.5 PA1 sodium oxide: 4.0 to 7.5 PA1 potassium oxide: 2.0 to 3.5
Composition of the borosilicate frit, in per cent by weight, is as follows:
However, tooling fabricated from this moulding composition is not strong enough owing to the relatively low adhesion between said borosilicate frit and corundum grains, the result of a low boron oxide concentration in said frit.
Another prior-art moulding composition to produce abrasive tooling (SU, A, 517203) comprises synthetic corundum material and bond. The bond comprises fireclay as a plasticizer, feldspar as an ingredient to increase bond strength properties, and lithium borosilicate frit and cryolite as reactive ingredients.
The proportions of the bond ingredients, in per cent by weight, are as follows:
Lithium borosilicate frit in this case has the following composition, in per cent by weight:
Tooling fabricated from this moulding composition is somewhat stronger than in the case of the previous counterpart, owing to the higher aggregate adhesive capacity of lithium borosilicate frit and cryolite relative to corundum grains.
However the lower melting temperature of the bond (ca. 800.degree. C.) comprised in this moulding composition and due to the presence of the low-melting ingredients of lithium and cryolite, will limit the hardness range of abrasive tooling that can be prepared from this moulding composition. Besides, as cryolite contains about 50% fluorine, harmful substances are evolved into the environment in fairly large amounts in the abrasive tooling production process. One other disadvantage of this moulding composition is that it is not readily available and costs rather high--because of the lithium borosilicate frit contained therein, an expensive product and in short supply.
There is known a moulding comopasition to produce abrasive tooling (SU, A, 931446), with the composition, in per cent by weight, as follows:
The adhesive and moistening ingredient is purposed to impart mouldability to the moulding composition.
The adhesive and moistening ingredients that can be used in moulding compositions include water glass and paraffin emulsion. The term "water glass" denotes the widely known aqueous sodium silicate solution. The paraffin emulsion formulations usable in moulding compositions are generally known.
The incorporation of barium silicate frit into the moulding formulation leads to higher melting temperature for the moulding composition, thus to a broader hardness range for the abrasive tooling, compared to the previous counterpart, yet the strength of the abrasive tooling produced from this moulding composition still remains insufficiently high. Besides, this moulding composition is not readily available and relatively costly, just as the previous counterpart. And harmful substances are evolved into the environment in rather large amounts in the process of fabricating it into abrasive tooling.